Method and apparatus for compressing and cooling air

ABSTRACT

The invention relates to a method for compressing and cooling air upstream from a facility for cryogenic separation of air, with no means for exchange by indirect heat, in which humid air is compressed in a compressor, the air compressed in the compressor is cooled in a first exchanger having indirect heat exchange, water is recovered from the air upstream and/or downstream from cooling in the first exchanger, the air cooled in the first exchanger is sent to a purification unit in order to produce air purified of carbon dioxide and/or water, the purified air is sent to the cryogenic separation facility, and at least one gas is recovered from the cryogenic separation facility.

The present invention relates to a process and to an apparatus for compressing and cooling air by indirect heat exchange.

In a cryogenic separation apparatus, the air, after it has been compressed, must be cooled as much as possible before entering the purification cylinders, in order to reduce the sizes thereof. It is proposed to simplify the cooling system.

Use will be made of the gaseous products resulting from the cold box, preferably all of the gaseous products, in order to cool the wet air, in a heat exchanger. A portion of the water condensed during the cooling of air in the proposed system, and also that resulting from the compression of the gas, will be injected into gases resulting from the cold box.

This system makes it possible to dispense with a dryer or a refrigerating unit, associated cooler problems, and optionally a water/nitrogen tower.

The heat exchanger will be able to be integrated into the main exchange line where the air is cooled to a cryogenic temperature upstream of the distillation.

After the compression of the air and the cooling thereof in the final cooler of the compressor (which may be replaced by the first stage of an air/water tower), it is possible to have:

-   -   a precooling system with water/nitrogen tower that generates         cold water, a refrigerating unit, an air/water tower or an         air/water heat exchanger;     -   a dryer and/or a direct air refrigerating unit;     -   nothing at all: there is then a purification referred to as hot         purification, which is of large size due to the amount of water         to be stopped, especially in summer. This large size may be         inconvenient if it is desired to put the cylinders in a sea         freight container.

According to one subject of the invention, a process is provided for compressing and cooling air upstream of a cryogenic air separation plant wherein:

-   -   wet air is compressed in a compressor;     -   air compressed in the compressor is cooled in a first heat         exchanger by indirect heat exchange;     -   water contained in the air is recovered upstream and/or         downstream of the cooling in the first heat exchanger, without         having cooled the air upstream of the first exchanger by a         cooling step by direct heat exchange;     -   air cooled in the first exchanger is sent to a purification unit         in order to produce air that is purified of carbon dioxide         and/or of water;     -   the purified air is sent to the cryogenic separation plant;     -   at least one, optionally heated, gas originating from the         cryogenic separation plant is recovered;     -   water recovered in step iii) is mixed with the gas originating         from the plant, the mixture produced being at a temperature         above the solidification point of the water in the mixture, for         example above 0° C. if the pressure of the mixture is close to         atmospheric pressure;     -   the water mixed with the gas is heated in the first heat         exchanger; and     -   if water contained in the air is recovered upstream of the first         heat exchanger, the air is cooled in a precooler upstream of the         recovery of water by indirect heat exchange.

According to other optional aspects:

-   -   the compression of step i) is isothermal;     -   the water mixed with the gas in the first heat exchanger is         mixed with the gas upstream of the first exchanger and/or at an         intermediate point of the first heat exchanger;     -   water contained in the air is recovered downstream of the         cooling in the first heat exchanger and upstream of the         purification unit;     -   the amount of water mixed with the gas is such that, at the         inlet point of the water mixed with the gas of the first heat         exchanger and/or at the intermediate point of the first heat         exchanger where the mixing takes place, the mixture of gas and         water is above the water saturation point;     -   water contained in the air is recovered upstream of the first         heat exchanger and the air is cooled in a precooler by indirect         heat exchange;     -   the gas is mixed solely with water produced by condensation of         water contained in the air;     -   the gas mixed with the water is nitrogen or oxygen;     -   at least one other gas, not mixed with water, is heated in the         first heat exchanger;     -   the gas mixed with the water is nitrogen and a stream of oxygen         and/or another stream of nitrogen is heated in the first heat         exchanger.

According to another subject of the invention, an air separation process is provided comprising a process for compressing and cooling air as claimed in one of the preceding claims, wherein the purified air is cooled in a second heat exchanger and sent to a column of the cryogenic separation plant, at least one product is withdrawn from the plant and heated in the second heat exchanger and then in the first heat exchanger and at least one product is withdrawn from the plant, heated, optionally vaporized, in the second heat exchanger and then, in gas form, in the first heat exchanger.

According to another subject of the invention, an apparatus is provided for compressing and cooling air intended for a cryogenic separation plant comprising a compressor, a first heat exchanger by indirect heat exchange, a purification unit, one or two separators, the compressor being connected to the first heat exchanger and the first heat exchanger being connected to the purification unit, the apparatus not comprising means of cooling air by direct exchange upstream of the first heat exchanger, the purification unit being suitable for sending purified air to the plant, the separator(s) being connected

i) between the compressor and the first heat exchanger, downstream of a precooler suitable for cooling the air by indirect heat exchange; and/or ii) between the first heat exchanger and the purification unit, at least a first line for conveying gas originating from the plant to the first heat exchanger, at least a second line for conveying water from the separator to the first line and/or the heat exchanger in order to mix water with the gas.

Optionally, the apparatus comprises:

-   -   a pump connecting the first heat exchanger with the separator;     -   a line for sending a dry gas heated in the first heat exchanger         to the purification unit;     -   lines for sending at least one other gas from the plant to the         first heat exchanger in order to be heated therein.

According to another subject of the invention, a plant is provided for the cryogenic separation of air by distillation, comprising an apparatus as claimed in one of claims 11 to 14, a system comprising at least one column, a second heat exchanger, a line for conveying air from the second heat exchanger to a column of the system and a line for conveying a distillation product to the second heat exchanger.

The invention is described in the case of a nitrogen generator, but may be extrapolated to other types of generators.

The air 1 is sent to a compressor 3. The compressor 3 may be an isothermal compressor, but the example shows an adiabatic compressor. The air compressed to 5 to 9 bar absolute and at 320° C. is cooled in a compressor cooler 6 by indirect heat exchange, to 27° C., then the water condensed is separated in a first separator 7, which may be integrated into the cooler of the compressor. The condensed water 11 is sent to a buffer tank 25. The air 9 is cooled in a first indirect heat-exchange heat exchanger 13, for example a plate-fin heat exchanger, against all the gaseous fluids 37, 39, 41 resulting from the cold box 33. The gaseous fluids are heated in the first heat exchanger 13 to 21° C. The water 19 condensed in the second separator 15 is sent to the buffer tank 25. The air 17 thus cooled to 10° C. then goes to the overhead purification 45.

The gas production 37, 43 is sent to the countercurrent heat exchanger.

In the case of the example, the stream 37 is pure gaseous nitrogen and the stream 43 is rich vaporized liquid originating from the overhead condenser of a single column for producing nitrogen, illustrated schematically. It will be readily understood that in the case of an application having a double column, the stream 37 could be gaseous oxygen or gaseous nitrogen and the stream 43 would be more or less pure gaseous nitrogen.

The residual fluid 43 at 10° C. and at atmospheric pressure (apart from the pressure drop from equipment downstream) is split into two portions 39, 41: one portion 41, which remains dry, which is used for regeneration of the purification 45 after heating in the heater 42, another portion 39, which will be humidified and cooled by injecting water 29, 31 resulting from the condensates. All the water contained in the gas 39 originates from the condensation of water in the air, either upstream or downstream of the first heat exchanger (13).

The injection of water may be carried out either completely before the heat exchanger (stream 29), or partly before (stream 29) until saturation of the gas, then along the heat exchanger (stream 31), optionally with several injection points. The injection could optionally be carried out using a pump 23, or directly using the pressurized condensates. The buffer tank 25 may be optional.

The separator 7 or the separator 15 may be eliminated, for example by being integrated into another piece of equipment.

The amount of water injected so as to be at most saturated on exiting the heat exchanger 13 will be limited to avoid sending water droplets into the atmosphere. This amount could be evaluated by calculation as a function of the operating parameters of the heat exchanger.

The chosen temperature of the air 17 at the outlet of the heat exchanger 13 before purification will be such that the temperature of the fluid that is saturated (or supersaturated) with water at the heat exchanger fluid inlet remains positive, to avoid the risk of ice formation and therefore of blockage.

The water used will originate from the ambient air (humidity) recovered in the condensates 11, 19. No particular treatment of these condensates will be needed.

There will be no permanent make-up of water from the outside to the system, only if necessary during very dry periods. It could also be envisaged to store rainwater, which would simply be filtered before injection into the system, during very dry periods.

The first air-cooling exchanger 13 may be integrated into the main exchange line 35 (second heat exchanger) of the cold box where purified air 17 originating from the purification 45 is cooled upstream of the distillation columns and where the residual nitrogen 43 is heated. The compressed air will then be withdrawn at an intermediate level of the integrated heat exchanger (13 and 35), then sent to the purification before returning to the exchange line, and the water will be injected into the fluid to be saturated along the top part of the heat exchanger. The introduction of the liquid could be carried out according to conventional techniques for injecting liquid into a gas in a plate heat exchanger, typically used for example for two-phase introductions.

In the case where the two heat exchangers 13, 35 are integrated, it is possible to envisage a heat exchanger, one part of which corresponding to the heat exchanger 13 would jut out from the wall of the cold box, the rest of the heat exchanger being inside the cold box.

The absence of cooling by expensive and bulky direct heat exchange will be noted in the process according to the invention. Likewise, it will be noted that the gas mixed with water is not used in a water cooling tower and only the water contained in the feed air is recovered. 

1-16. (canceled)
 17. A process for compressing and cooling air upstream of a cryogenic air separation plant, comprising: i. compressing wet air a compressor; thereby producing a compressed air stream, ii. cooling the compressed air stream in a first heat exchanger by indirect heat exchange; thereby producing a cooled air stream comprising water, iii. recovering the water contained in the cooled air stream upstream and/or downstream of the cooling in step ii), without having cooled the air upstream of the first exchanger by a cooling step by direct heat exchange; thereby producing a dry air stream, iv. sending the dry air stream to a purification unit in order to produce a purified air stream that is purified of carbon dioxide and/or of water; v. sending the purified air stream to a cryogenic separation plant; thereby producing at least an oxygen rich stream and an oxygen depleted stream, vi. recovering at least one of the oxygen rich stream or the oxygen depleted stream; vii. mixing the water recovered in step iii) with the gas removed in step vi), the mixture produced being at a temperature above the solidification point of the water in the mixture; viii. heating the water mixed with the gas in the first heat exchanger; and ix. cooling the air recovered upstream of the first heat exchanger in a precooler upstream of the recovery of water by indirect heat exchange, if water contained in the air is recovered upstream of the first heat exchanger.
 18. The process of claim 17, wherein the compression of step i) is isothermal.
 19. The process of claim 17, wherein the oxygen rich stream or the oxygen depleted stream recovered instep vi) is heated.
 20. The process of claim 17, wherein the production temperature in step vii) is above 0° C. if the pressure of the mixture is close to atmospheric pressure.
 21. The process of claim 17, wherein the water mixed with the gas in the first heat exchanger is mixed with the gas upstream of the first exchanger) and/or at an intermediate point of the first heat exchanger.
 22. The process of claim 17, wherein water contained in the air is recovered downstream of the cooling in the first heat exchanger and upstream of the purification unit .
 23. The process of claim 17, wherein the amount of water mixed with the gas is such that, at the inlet point of the water mixed with the gas of the first heat exchanger and/or at the intermediate point of the first heat exchanger where the mixing takes place, the mixture of gas and water is above the water saturation point.
 24. The process of claim 17, wherein water contained in the air is recovered upstream of the first heat exchanger and the air is cooled in a precooler by indirect heat exchange.
 25. The process of claim 17, wherein the gas is mixed solely with water produced by condensation of water contained in the air.
 26. The process of claim 17, wherein at least one other gas, not mixed with water, is heated in the first heat exchanger.
 27. The process of claim 17, wherein the gas mixed with the water is nitrogen and a stream of oxygen and/or another stream of nitrogen is heated in the first heat exchanger.
 28. An air separation process comprising a process for compressing and cooling air as claimed in one of the preceding claims, wherein the purified air is cooled in a second heat exchanger and sent to a column of the cryogenic separation plant, at least one product is withdrawn from the plant and heated in the second heat exchanger and then in the first heat exchanger and at least one product is withdrawn from the plant, heated, optionally vaporized, in the second heat exchanger and then, in gas form, in the first heat exchanger.
 29. An apparatus for compressing and cooling air intended for a cryogenic separation plant comprising a compressor, a first heat exchanger by indirect heat exchange, a purification unit, one or two separators, the compressor being connected to the first heat exchanger and the first heat exchanger being connected to the purification unit, the apparatus not comprising means of cooling air by direct exchange upstream of the first heat exchanger, the purification unit being suitable for sending purified air to the plant, the separator(s) being connected i. between the compressor and the first heat exchanger, downstream of a precooler (6) suitable for cooling the air by indirect heat exchange; and/or ii. between the first heat exchanger and the purification unit, at least a first line for conveying gas originating from the plant to the first heat exchanger, at least a second line for conveying water from the separator to the first line and/or the heat exchanger in order to mix water with the gas.
 30. The apparatus of claim 29, wherein the pump connecting the first heat exchanger with the separator.
 31. The apparatus of claim 29, further comprising a line for sending a dry gas heated in the first heat exchanger to the purification unit.
 32. The apparatus of claim 29, further comprising lines for sending at least one other gas from the plant to the first heat exchanger in order to be heated therein.
 33. A plant for the cryogenic separation of air by distillation, comprising an apparatus of claim 29, wherein a system comprising at least one column, a second heat exchanger, a line for conveying air from the second heat exchanger to a column of the system and a line for conveying a distillation product to the second heat exchanger. 